Sensors and Actuators B 121 (2007) 208–213
Synthesis and characterization of semiconducting
nanowires for gas sensing
G. Sberveglieri
∗
, C. Baratto, E. Comini, G. Faglia, M. Ferroni,
A. Ponzoni, A. Vomiero
SENSOR Lab of CNR-INFM and Dipartimento di Chimica e Fisica per l’Ingegneria e per i Materiali,
Brescia University, via Valotti 9, 25133 Brescia, Italy
Available online 27 October 2006
Abstract
Quasi one-dimensional nanostructures of semiconducting metal oxides are promising for the development of nano-devices. Tin, indium, and zinc
oxides were produced in form of single-crystalline nanowires through condensation from vapor phase. Such a growth occurs in controlled thermo-
dynamical condition and size reduction effects on the electrical and optical response to gases have been exploited. Preparation, microstructural,
and electrical characterization of nanowires are presented and the peculiarities of these innovative structures are highlighted.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Nanowires; SnO
2
;In
2
O
3
; ZnO; Ozone
1. Introduction
A new generation of nanostructures has been recently pro-
duced and has attracted the interest of a wide research com-
munity [1]. These fascinating quasi one-dimensional nanostruc-
tures, namely nanowires, nanorods, and nanobelts, exhibit a
single-crystalline arrangement and feature unusual electrical and
optical properties, which arise from size reduction or quantum
confinement as crystal size is comparable to the wavelength of

faces, and defects. Unfortunately, thermal treatment promotes
grain coarsening and causes degradation of the functionality by
suppressing the surface-to-volume ratio [9].
Differently, newly developed quasi one-dimensional nanos-
tructures envisage long durability owing to their exceptionally
high degree of crystallinity [10]. The transverse dimension of
nanowires may result even smaller than the Debye length asso-
ciated to the surface space-charge region and in such condition
the detection efficiency of gas molecules adsorbedatsurface may
reach very high value [11]. This extraordinary sensing potential
has been recently demonstrated for operation in liquid envi-
ronment or at room temperature [12–14]. Among the possible
applications in the field of bio-nanotechnology, sensitive DNA
and protein detection are presently under investigation [15].
This paper summarizes the preparation and characterization
of tin, zinc and indium oxide nanowires. The electrical and
0925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.snb.2006.09.049
G. Sberveglieri et al. / Sensors and Actuators B 121 (2007) 208–213 209
Table 1
Basic operating parameters for nanowire growth from vapor condensation
Nanowires Precursor Decomposition
temperature (
◦
C)
Substrate
temperature (
◦
C)
Duration (min) Pressure (mbar) Substrate Catalyst

Absent
optical properties of nanowires were investigated with particular
regard to gas sensing behavior.
2. Experimental
The growth of MOX nanowires from vapor phase is based on
the evaporation–condensation technique [10]. The oxide pre-
cursor powder is placed at the center of an alumina tube and
then temperature is raised above the limit of decomposition for
the oxide (from 600
◦
C for zinc oxide to 1500
◦
C for indium
oxide) [16]. A controlled flow of inert gas (usually argon) is
maintained during decomposition and the overall pressure mea-
sures hundreds of mbar. The temperature gradient downstream
the gas flow promotes condensation of cations on clean alumina
substrates and allows interaction with the residual oxygen. The
peculiar thermodynamic conditions promote growth of nano-
sized one-dimensional structures instead of equi-axed grains.
Fig. 1 shows the nucleation of indium oxide nanowires as
achieved by the evaporation–condensation process. The SEM
image shows the crystal habit for the nanowires: the section
appears to be squared and the apex of the wires is tapered. In
Fig. 1. Nucleation of indium oxide nanowires over polycrystalline alumina.
general, no epitaxial relationship between the orientation of the
wire and the alumina grains has been observed. Control over the
direction of growth as well as pattering of the substrates may be
achieved by assisting the growth mechanism through dispersion
of catalysts [17].

than 100 nm. As shown in Fig. 2a, the length and width of the
nanowire measure 25.3 ␮m and about 50 nm, respectively. The
length and flexibility allows nano-manipulation for removal and
positioning over Si-based substrates for functional characteriza-
tion (see Fig. 2b). High-resolution TEM and electron diffraction
showed that the wire is single crystalline, with atomically sharp
termination of lateral sides. Measured Bragg reflections and the
whole symmetry of the ED pattern (see Fig. 2c) agree with the
cassiterite tetragonal SnO
2
phase (P42/mnm - SG 136). The
direction of the electron beam is parallel to the [0 1 0] zone-axis
of the reciprocal lattice and the nanowire grows along to the
[1 0 0] direction.
210 G. Sberveglieri et al. / Sensors and Actuators B 121 (2007) 208–213
Fig. 2. Characteristics of SnO
2
nanowires: (a) low-magnification TEM image of a very long SnO
2
nanowire, (b) removal of nanowires from the alumina substrate
through manipulators for structural and electrical characterization, (c) ED pattern of nanowire, (d) STEM-HAADF image of a nanowire, and (e) linescan of the
HAADF signal (solid line) and numerical fit of the shape of the nanowire (dashed line).
As both composition and phase can be considered uniform for
the crystalline SnO
2
nanowire; STEM-HAADF directly visu-
alizes variations in the projected thickness. Fig. 2d shows a
STEM-HAADF image of a SnO
2
nanowire, about 45 nm in

2
O
3
and that the growth direction is par-
allel to the [1 0 0] direction.
3.3. ZnO nanowires
ZnO nanowires may be produced at relatively low decom-
position temperature (see Table 1); the size and shape of the
obtained nanowires is however sensitive to the condensation
condition. Fig. 4 shows that ZnO nanowires smaller than 10 nm
in width can be produced. The capability to control the lateral
dimension of the nanowires will allow the systematic investiga-
tion of size reduction effects on the electrical and gas sensing
behavior of ZnO nanowires.
Fig. 3. Characteristics of In
3
O
2
nanowires: (a) SEM image of In
3
O
2
nanowires, (b) TEM image of nanowire 70 nm in width, and (c) ED pattern from the nanowire.
G. Sberveglieri et al. / Sensors and Actuators B 121 (2007) 208–213 211
Fig. 4. Characteristics of ZnO nanowires: (a–c) variation of the size for the ZnO nanowires for different growth conditions, (d) TEM image of ZnO crystalline
nanowire, (e) high-resolution TEM image of the hexagonal nanowire lattice, and (f) digital diffractogram and sketch of the indexed Bragg reflections.
TEM observation confirms the regular crystalline arrange-
ment for the nanowires. No evidences of extended crystal defects
governing the growth have been recorded. The high-resolution
TEM image and the corresponding digital diffractogram indicate

encountered for conventional MOX-based sensors in sensing of
oxidizing species [19].
ZnO nanowires exhibit low response to ozone. By observ-
ing the dynamic of response, three processes with different
time constant can be observed: a quick decrease in conductance
occurred after the ozone injection and is followed by a conduc-
tance increase; finally a very slow process prevented the sensor
response from reaching a steady-state value even after 1 h from
ozone injection. Despite this phenomenon, the response keeps
reversible.
The high response of the nanowires can be attributed
to their small lateral dimension. Indeed, when the lateral
dimensions of the nanowire are sufficiently reduced, then the
nanowire can be completely depleted and the response to gases
increases [20].
Fig. 6 shows the ozone sensing capability of SnO
2
and In
2
O
3
nanowires as a function of the operating temperature. ZnO
nanowires are not reported because of their slow response. The
highest response is obtained for an operating temperature of
400
◦
C for both the samples.
5. Optical characterization
Photoluminescence (PL) spectroscopy was performed over a
wide temperature and wavelength range for the purpose of inves-

(dashed line)
as a function of the operating temperature. Ozone concentration is 280 ppb.
Fig. 7. Spectrum of photoluminescence at room temperature for ZnO nanowires
in dry air (open squares), 20 min after NO
2
introduction (open triangles) and
20 min after dry air restoration (open circles).
effect is fast (time scale order of seconds) and fully reversible.
The amplitude of quenching achieves its maximum at room
temperature and the influence of humidity and other reducing
gases is negligible. This feature could be interesting for applica-
tion of nanowires as a selective optical sensor working at room
temperature.
6. Concluding remarks
Nanowires of semiconducting MOX can be effectively pro-
duced through evaporation–condensation process. Control over
the size of the nanowires is achieved by proper modification
of the operating conditions. Nanowires of SnO
2
,In
2
O
3
and
ZnO have been produced in their stable and common crystalline
phase.
The high degree of crystallinity and the small lateral dimen-
sion of these quasi 1D nanostructures open the perspective of a
new class of stable nano-devices for gas sensing.
Acknowledgements

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Biographies
G. Sberveglieri was born on 17 July 1947 and received his degree in physics cum
laude from the University of Parma (Italy), where he started in 1971 his research
activities on the preparation of semiconducting thin film solar cells. He is now
the director of the CNR, INFM Sensor Laboratory (
)
at Brescia University where more than 20 researchers are working. In 1988 he
established the Gas Sensor Lab, mainly devoted to the preparation and char-
acterization of thin film chemical sensors based on nanostructured metal oxide
semiconductors and, since the mid 1990s, to the area of electronic noses. In
1994, he was appointed full professor in physics. He is referee of many inter-
national journals and associate editor of IEEE Sensor Journal and has acted
as chairman in several Conferences on Materials Science and on Sensors. He

referee.
M. Ferroni received his PhD degree in physics at the University of Ferrara in
1998, and became researcher at the University of Brescia in 2004. His mean
research activity concerns the characterization of nanostructured metal oxides
by means of transmission and scanning electron microscopy. Presently, Matteo
Ferroni is in charge of the high-resolution scanning electron microscopy facility
at the CNR-INFM SENSOR laboratory in Brescia.
A. Ponzoni was born in 1976. He received the degree in physics from the Uni-
versity of Parma in 2000. In 2006, he received the PhD degree in material
engineering from the University of Brescia with a thesis on nanostructured metal
oxides for gas sensing applications. His main activity regards synthesis and elec-
trical characterization of metal oxides for gas sensing applications. Presently,
he is researcher at the CNR-INFM Sensor Lab, Brescia.
A. Vomiero received his degree in physics at the University of Padova in 1999,
and his PhD in electronic engineering at the University of Trento in 2003. His
main activities deal with the synthesis of thin films and nanostructured materials
by the means of PVD techniques and the application of low energy nuclear
techniques to materials science. Presently, he is researcher at the CNR-INFM
SENSOR Lab, Brescia.